Most optical links at present employ intensity modulation and direct detection. Due to the inherent nonlinearity inside the optical intensity modulation process, such links suffer from high nonlinear distortion, which leads to limited dynamic range. The state-of-the-art spurious free dynamic range, or SFDR, is currently limited to 115 dB·Hz2/3. This level of performance has been achieved by either linearizing the optical modulator or introducing pre-distortion.
Modern systems architectures often require the ability to ‘remote’ selected components from each other. For example, components such as sensors and the central processing unit (CPU), or the control unit and the phased array antenna are often not co-located. A data link provides the required connectivity that enables data flow between such components, becoming inadvertently an integral part of the system, and potentially a system performance limiting factor. Continued and ongoing research and development (R&D) efforts focus today on data links with higher dynamic range, higher data rates and bandwidth, and lower noise with an intent of making such links ‘transparent’, i.e., virtually unaffecting, largely ‘invisible’ to the primary system. Optical data links where the data is modulated on an optical carrier emanating from a source such as a laser and transmitted in either free space or in an optical medium (such as optical fiber or waveguide), are most commonly used.
In the current state-of-the-art, the data (in the form of electromagnetic signals) modulates the intensity of the optical carrier emanating from a laser source, travels along the optical link and is converted back to electromagnetic signals at the optical detector. The subsequent signal demodulation is done directly, typically through the use of a photo-mixer (such as a photodetector). One of the bottlenecks in such an optical link is the limited spurious free dynamic range (SFDR) due to the nonlinear distortion incurred within the modulation/demodulation processes. The current state-of-the-art manages to achieve SFDR in the range of 115 dB·Hz2/3 by linearizing optical modulators, linearizing photodiodes, or introducing pre-distortion. However, some of the critical applications, such as antenna remoting, require an SFDR of at least an order of magnitude higher, for optimal performance. It is for such applications that high quality angle modulated optical links (made possible by the PPLL device disclosed herein) can play a role.
Angle modulation, of which FM and PM are subsets, refers to the variation of the phase angle of a carrier signal in proportion to an information signal. FM is the modulation of the instantaneous frequency, which can be related to PM by defining the instantaneous frequency to be the time derivative of the phase. In traditional electronic systems, angle modulation has been preferred over amplitude modulation (AM) for several reasons, including greater immunity from noise and static, and the ability to improve the signal to noise ratio (SNR) by sacrificing additional bandwidth. Electronic systems employing angle modulation have enjoyed strong success due to these advantages.
However, these benefits are even more attractive in the optical domain for a variety of reasons. Modern optical communication systems using intensity modulation (essentially a variant of AM) suffer from relatively poor system link performance compared to purely electronic counterparts. Furthermore, the extremely high operating frequencies of optical carriers (at several orders of magnitude above microwave systems) make bandwidth an abundant resource which can be used to advantage.
Optical phase and frequency modulation has been demonstrated using a variety of techniques. In general, modulation techniques can be categorized as either being external to the optical source, or inside the optical source itself. These techniques include, but are not limited to, the use of laser free spectral range tuning elements and modulation of the optical path.
In electrical FM (and PM) systems, signal demodulation is normally achieved by either directly converting the frequency information back into amplitude (through the use of a frequency discriminator), or by using a second frequency modulating element inside of a control loop to indirectly generate a signal that appears similar to the original information. Note that most frequency discrimination techniques assume that the amplitude of the FM signal is constant. Thus, fluctuations of the amplitude (e.g., through static or noise) will distort the demodulated signal. Therefore, most electronic discriminators are preceded by amplitude limiters.
A simple example of a frequency discriminator would be the use of a differentiator circuit, followed by an envelope detection circuit. FM signals which are passed through a differentiator circuit (such as a simple CR differentiator, which consists of a capacitor and resistor) will be amplitude modulated in proportion to their frequency. An envelope detection circuit will then remove the frequency information, and give an output signal that is proportional to the original information.
A related FM demodulation technique is the slope detector frequency demodulator. In this technique, a filter whose transition band falls within the bandwidth of the FM signal is used. If the filter frequency response is a fairly linear function of frequency, the ensuing amplitude of the output will also be linearly proportional to the information signal. The linearity of this approach can be improved by subtracting the falling edge of a bandpass element from the falling edge of a band reject element, as is done in the case of a triple-tuned discriminator.
Another frequency demodulator is the Foster-Seeley circuit, which is tuned to the carrier frequency of the FM signal. Two complementary sub-circuits are tuned such that when the input is at the resonant frequency, the outputs of the sub-circuits cancel each other. The circuits exploit the linear phase response of circuits near resonance. Thus, when the carrier frequency is modulated away from the resonant frequency, there is a corresponding phase change which causes the sum of the sub-circuits to no longer completely cancel out. In effect, another frequency-to-amplitude conversion has been made.
A similar demodulator that was popular in radio systems of the past is the ratio detector. However, instead of using two diodes in opposing polarity to cancel each other, the diodes are placed in series. Ratio detectors were popular for their hardiness against amplitude noise, owing to the use of large capacitive elements to smooth out high frequency fluctuations. Nevertheless, this type of system has fallen out of disfavor due to the need for transformer elements.
The linear phase response of near-resonant circuits is also integral to the operation of quadrature detectors, which are still used today. The output signal is split, and one of the halves is put in quadrature (shifted in phase by 90 degrees) and passed through a resonant circuit. The two signals are then put in a frequency mixer, which is a nonlinear multiplication element. The mixed signal has an amplitude which is dependent on the frequency deviation (since this frequency deviation produces a nearly linear phase change in the resonant circuit), once again performing an FM-to-AM conversion.
Another commonly used system is the time delay discriminator. This system simply splits the signal and slightly delays one of the halves. The two are then recombined. If the delay between the paths is very small relative to the length of the paths themselves, the result is once again analogous to a differentiator, and may be handled as such.
Alternatively, instead of differentiating the FM signal, the zero crossings of the signal can be counted instead. At higher frequencies, the FM signal amplitude will be zero more often than at lower frequencies. Through the use of a monostable pulse generator, a pulse will be produced at each zero crossing of the FM signal. Integrating these pulses yields an output which is proportional to the original information.
The phase-locked loop (PLL) is popular in high quality FM systems. It is an indirect method of demodulating FM signals, in that the loop is designed so that a voltage controlled oscillator (VCO) inside the loop tracks the frequency of a locking master signal. The generation of an error voltage which is proportional to the original signal is a consequence of this locking mechanism.
Typically, the input signal to a PLL is introduced to a phase comparator element, which generates an error voltage whose amplitude is proportional to the phase difference between the input and a VCO. The error voltage is then applied to the VCO, whose frequency is adjusted in proportion to the voltage amplitude. This forces the phase and frequency of the VCO inside the loop to be continuously synchronized to the original.
A variant of the PLL used for FM demodulation is the FM demodulator with feedback (FMFB). Instead of a phase comparator, the FMFB uses a frequency mixer, filter, and discriminator.
Similar progress in optical (as opposed to electronic) links has been somewhat slow. The predominant method to date involves direct modulation of the optical intensity to transmit information. This is done even when the original signal is angle modulated or digitally keyed, by intensity modulating the optical carrier with the entire electronic spectrum, in a process known as sub-carrier modulation (SCM). Quality and performance suffers in these approaches, in terms of spurious free dynamic range (SFDR), insertion loss, and noise figure (NF). The advantage is the conceptual simplicity of direct intensity modulation at the transmitter and receiver, as well as low number of components (and therefore, low cost).
To date, some basic research has been conducted in the use of optical angle modulation as an alternative transmission scheme. Nakajima has demonstrated FM and FSK (i.e., digital information encoded through the use of FM) demodulators by using injection locking of distributed feedback (DFB) lasers. Nakajima, H., “Demodulation of multi-gigahertz frequency-modulated optical signals in an injection-locked distributed feedback laser oscillator,” Electronics Letters, vol. 26, no. 15, Jul. 19, 1990, pp. 1129-1131 Nakajima et al., “Direct demodulation of 140 Mb/s FSK signals in an injection-locked multiquantum-well DFB laser,” IEEE Photonics Technology Letters, vol. 3, no. 11, November 1991, pp. 1029-1031. The demodulator relies on a laser which is injection locked to the optical signal from the transmitter. It performs an FM-to-AM conversion because the quasi-Fermi level separation inside the DFB laser is a function of the frequency detuning of the injected frequency. Thus, as the optical injection frequency is detuned from the locking frequency center, there is a measurable electrical voltage change across the laser device. More recently, Chen, et al. have shown an optical FM discriminator, where demodulation is performed with an optical analogue of a slope detection circuit. Chen et al., “Frequency Discriminator Based on Ring-Assisted Fiber Sagnac Filter,” IEEE Photonics Technology Letters, vol. 17, no. 1, January 2005, pp. 109-111. However, both of these approaches cannot capitalize on the advantages of optical FM links, because the demodulation schemes each require a preliminary FM-to-AM conversion in order to recover the original information. Thus, both schemes are inherently limited by the same ultimate deficiencies as the ubiquitous direct intensity modulation schemes, which are already AM approaches.
Since it is an improvement of electronic PLL demodulation, which is an indirect method, angular demodulation using the PPLL does not suffer from this limitation and is capable of supporting links that have high dynamic range, low insertion loss, low noise figure, and high linearity. Although other schemes to directly perform angular demodulation in the optical domain have previously been implemented, their limitations have not allowed them to achieve performance that significantly surpasses the current state-of-the-art intensity modulated, direct detection optical link.
The proposed invention relates to an alternative type of optical link employing coherent optical angular modulation (as opposed to intensity modulation), now made possible as a superior alternative to intensity modulated links by a fundamentally new, intrinsically linear photonic PLL (PPLL) discriminator (as opposed to an incrementally new linearization technique). The invention being disclosed presents the concept and a design of this novel Photonic PLL enabling FM or PM signal demodulation in the optical domain, resulting in a significant performance improvement of the optical link, and more specifically, improvement of its dynamic range (with SFDR typically exceeding 150 dB·Hz2/3).
The novel PPLL demodulator disclosed here makes a high quality, angle modulated optical link (with performance surpassing intensity modulated links) possible for the first time. A microwave fiber link employing such a PPLL demodulator is able to yield an SFDR larger than 160 dB·Hz2/3 